A motor control center (MCCs) typically includes a cabinet or enclosure that is configured to hold “bucket” assemblies or units, which may be configured in a modular fashion, such as shown in U.S. Pat. No. 4,024,441. Such units may contain various motor control and motor protection components such as motor controllers, starters, contactor assemblies, overload relays, circuit breakers, motor circuit protectors, and other devices. The units may be configured to connect to power buses of the motor control center and control provision of power to motors or other devices.
Some MCC units may be configured such that, when installed in the MCC, bus contacts of the unit may be releasably engaged with the power buses of the MCC. This allows the MCC unit to be de-energized without removing the unit from the MCC. Such MCC units are described, for example, in U.S. Patent Application Publication No. 2008/0022673.
As noted above, MCC units typically include control circuitry that is used to control motors and other devices. Such control circuitry is often powered from the MCC buses via step-down transformers that convert from the MCC bus voltage (e.g., 480 VAC) to the control circuit voltage (e.g., 120 VAC). It is generally desirable to be able to test the operation of such control circuitry to, for example, assure proper operation upon installation, perform preventive maintenances and troubleshoot malfunctions. Such testing may be conducted in situ and/or at a test bench.
If testing is performed in situ, it is desirable to perform such testing with the MCC unit control circuitry disconnected from the MCC power buses. Some MCC units are configured to allow power to be provided to the MCC unit control circuitry from an external source to support testing of the control circuitry when that control circuitry is not powered by the MCC power bus. For example, the MCC unit control circuitry may be powered using a pigtail connection and/or by connecting an external power source to the MCC unit control circuitry using a power input socket. An MCC unit may include a control power transformer that has a primary connected to bus contact stabs of the MCC unit and a secondary that may be coupled to the control circuit power input socket using a jumper cable that must be unplugged in order to connect the control circuitry to an external power source, thus preventing connection of the external power source to the control power transformer and dangerous backfeed to the MCC bus. However, such techniques may require opening the door of the MCC compartment containing the MCC unit, creating a possibility that an operator may be exposed to the MCC bus voltage of the MCC bus and, thus, requiring the operator to don personal protection equipment (PPE) to meet safety requirements. Accordingly, there is a need for improved techniques for providing test power to MCC units.